The present invention relates to a method and a device for facilitating the sliding, along a track, of an element having a predominant axial dimension subjected to an intense force pressing it against the track, particularly for facilitating the sliding of a wire subjected to stranding in stranding machines or the like.
It is known that in many machines used to perform the stranding of wires in general or of conducting wires for electrical systems or for telecommunications, a wire is guided along a stranding arc, known as bow, which is fixed at its ends to two symmetrical flyers, each of which is fixed to a shaft of a pair of coaxial shafts which are actuated, rigidly with respect to each other, with a rotary motion about their common axis.
In practice, during the stranding operation, a wire moves along the stranding bow while the bow is actuated with a rotary motion about an axis which passes through the ends of the bow. The advancement of the wire along the stranding bow is produced by a traction force which is applied to the wire downstream of the stranding bow along the wire advancement direction.
During the rotation of the stranding bow, the wire that advances along the bow, due to the centrifugal force produced by the rotation of the bow, is pressed against the side of the bow that is directed toward the rotation axis. On this side of the bow there is a metal sliding track which has a reduced friction coefficient with respect to the wire that must advance along it.
The productivity of these machines is a function of the rotation rate of the bow, which cannot exceed a certain limit which in turn is a function of the maximum traction that can be applied to the wire downstream of the stranding bow. As the rotation rate of the stranding bow increases, the centrifugal force increases the pressure applied by the wire to the sliding track and accordingly the friction force that contrasts the advancement of the wire along the bow increases. This increase in the friction force requires, in order to achieve the advancement of the wire, an increase in the traction applied to the wire downstream of the stranding bow, which however cannot exceed a maximum value if one wishes to avoid causing damage or modifications of the wire that are not compatible with its subsequent use. For these reasons, stranding machines, particularly machines for stranding signal conductors for control and communication, i.e., wires with a copper core covered with an insulating layer having a low mechanical strength, currently cannot reach high rotation rates of the stranding bow and therefore have rather low productivities.
Merely by way of example, in a conventional stranding machine for stranding telecommunications cables, if one wishes to limit the drawing traction that can be applied to the cable, Tmax, to the 30 N required to preserve the electrical and insulation characteristics of the cable that are currently required, assuming that the portion of wire contained along the entire path of the bow has a mass m=2xc2x710xe2x88x922 kg, a friction coefficient fa=0.25, an average radius of the rotation path of the bow R=0.225 m, and requiring the sliding friction force along the bow to be lower than, or equal to, the maximum applicable traction, one obtains:
Tmax=30Nxe2x89xa6faxc2x7mxc2x7Rxc2x7(2xcfx80n)2=0.25xc2x72xc2x710xe2x88x922xc2x70.225xc2x740 n2 
where n=rotation rate of the bow in revolutions per second from which:
n2xe2x89xa7666 and therefore nxe2x89xa726 revolutions/second=1,550 rpm i.e., an excessively low speed with respect to modern production requirements.
The aim of the present invention is to solve the above-noted problem, by providing a method and a device which allow to facilitate the sliding, along a track, of an element having a predominant axial dimension, or wire-like element, which is subjected to an intense force pressing it against the track, particularly for facilitating the sliding of a wire subjected to stranding in stranding machines or the like.
Within this aim, an object of the present invention is to provide a method and a device which, particularly in stranding machines, allow to increase the rotation rate of the stranding bow and therefore to increase the productivity of these machines.
Another object of the invention to provide a method and a device which, by facilitating the sliding of the wire along the track, allow to maintain the traction force, to be applied to the wire in order to produce its advancement, below the limit value in order to ensure high wire quality.
This aim and these and other objects which will become better apparent hereinafter are achieved by a method for facilitating the sliding, along a track, of an element having a predominant axial dimension subjected to an intense force pressing it against the track, particularly for facilitating the sliding of a wire subjected to stranding in stranding machines or the like, characterized in that it comprises the step of interposing between said element and said track a pressurized fluid which contrasts the force that presses said element against said track.